166 RADIATION BIOLOGY 



is also one of the necessary constants in the evaluation of dosage, it might 

 be well to call attention to some recent developments. 



Since the theory of the stopping power breaks down at low velocities 

 and it seems unlikely to be improved in that region,^ particularly because 

 of electron capture and loss by slow-charged particles (Bethe, 1950), it 

 seems rather urgent that in addition either W or pm, or preferably the 

 prodvict Pm X W, be determined experimentally for recoil nuclei of bio- 

 logical interest (H^ C^", N^^, 0^") in gas mixtures simulating tissue com- 

 position. The situation calls for more than academic interest, since 

 comparison of the careful data of Rossi (1949) on uranium a particles 

 with those of Curran et at. (1950) with ^ rays of H^ show excellent agree- 

 ment in the ratio Wx/W^ii only for the components of air (as is to be 

 expected) but wide variation (up to 19 per cent) in polyatomic gases of 

 atomic composition different from air. The difficulty of determining 

 D = JpmW cannot be resolved simply by making p„ = 1 by the proper 

 choice of gas mixture since the variation of Wx in this gas mixture as a 

 function of energy, mass, and charge of the ionizing particle would have 

 to be determined de novo. The alternative remains of choosing a noble 

 gas such as helium (for which W is supposedly constant for all ionizing 

 particles of any energy) and of determining the stopping power ratio pm 

 with respect to it. This procedure does not avoid the tedious and pains- 

 taking task of determining the ratio of stopping power between a gas and 

 a solid and, therefore, does not promise immediate advantages since it 

 would make the linear dimensions of the cavity of critical importance in 

 many measurements. 



Finally, the stopping power of matter for high-speed particles (elec- 

 trons > 1 Mev, mesons > 200 Mev) is considered to be affected by the 

 polarization in soUd media. The theoretical corrections predicted 

 (Halpern and Hall, 1948; Fermi, 1940; Wick, 1943) seem to have found 

 substantiation in experiments (Hereford, 1948; Bowen and Roser, 1951) 

 recently reported. It seems evident, therefore, that the values of p„ (or 

 B) as derived from the Bethe-Bloch formulas by Laurence should be 

 reconsidered and checked experimentally under those conditions of 

 irradiation which are of interest to the biologist. 



Of immediate interest to this topic also is the finding by Appleyard 

 (1951) that the relative stopping power of liquid water for a particles of 

 4 to 5 Mev energy is about 13 per cent greater than that calculated on the 

 basis of its atomic constitution and that, for the same radiation, W^o 

 (vapor) is 0.88Tfair (Appleyard, 1949). 



DOSE FROM RADIOELEMENTS DEPOSITED IN BIOLOGICAL SYSTEMS 



Beta-ray Emitters. The irradiation of biological or chemical systems 

 by /3-ray emitting elements deposited therein offers, sometimes, practical 

 9 See, however, Neufeld, 1950. 



